The present invention relates to an overhead projector, and more particularly, to an overhead projector which initially forms an image corresponding to an object or the contents of a sheet of paper by employing a specific wavelength and enlarges the image without affecting the projected image.
In general, an overhead projector is used to project an enlarged image recorded on a film onto a projection screen. FIG. 1 shows a conventional overhead projector 10 which includes a case 12 whose upper surface comprises a Fresnel lens 11, a mirror 14 installed in an interior of case 12 by a predetermined angle and which faces Fresnel lens 11, an optical source 15 installed in one side of an interior of case 12 and which irradiates light onto mirror 14, a focusing lens 13 installed between optical source 15 and mirror 14, an image forming lens group 17 located above Fresnel lens 11 and supported by a support 16 fixed to one side of case 12, and a reflection mirror 18 installed on an optical axis of image forming lens group 17.
When the conventional overhead projector 10 is used, a film 100, having a predetermined image printed thereon is placed on Fresnel lens 11 at the state where light is irradiated from optical source 15. The light irradiated from optical source 15 is reflected onto mirror 14 and changed into focusing light while passing through Fresnel lens 11. Then, the light changed into focusing light passes through film 100 and is irradiated onto a screen via image forming lens group 17, thereby obtaining an enlarged image.
The conventional overhead projector 10 has an advantage in that the image formed on a screen is clear and illuminance is high. However, actual objects and the contents of a sheet of paper cannot be used directly, which means that an additional film has to be prepared. When the film 100 is prepared, the projection process is simple. However, since the film 100 has to be of a size corresponding to the contents of paper, it is difficult to enhance optical efficiency. Optical efficiency can be achieved via the use of a larger Fresnel lens, but this increases manufacturing costs significantly.
For another conventional projector 10' attempting to solve the above-described problem, as shown in FIG. 2, an image forming lens group 22 is installed at one side of case 21 and a mirror 23 is installed at a predetermined angle in an interior of case 21 facing the image forming lens group 22. A transparent supporting plate 24 is provided at the upper surface of case 21 so as to support the paper 100, and a plurality of optical sources 25 that irradiate light onto paper 100 supported by supporting plate 24 are installed within case 21.
For the conventional overhead projector 10' of FIG. 2, when the light is irradiated from optical source 25 onto paper 100, the light is reflected from the paper. The reflected light is reflected onto mirror 23, and thus, an enlarged image is formed onto a projection screen via image forming lens group 22.
However, the conventional projector 10' has certain problems.
First, the reflectance of the light irradiated from optical source 25 onto paper 100 and reflected from the paper 100 is approximately 17%. Therefore, a strong optical source is needed in order to obtain a clear image, which causes problems in terms of radiation and the power supply required.
Second, since the surface illuminance of the paper 100 supported by the supporting plate 24 is poor, the light irradiated from an optical source 25 is irregularly reflected and thus results in a small amount of usable light. Therefore, light from an extremely strong optical source has to be irradiated onto the paper or object.
Third, since a high luminance optical source and facilities therefor are needed, the size and price of an overhead projector containing the same are increased accordingly.